Light-emitting diodes (LEDs) are widely used in screen backlighting. Usually multiple LED strings are needed to light the screen. Nevertheless, in practical applications, different voltages are needed when each of the LED strings comprises a different number of LEDs. For example, a first LED string comprises 10 LEDs and a second LED string comprises 20 LEDs, then two different voltages are needed to respectively supply the first LED string and the second LED string. A common solution is using two independent switching circuits to produce two different voltages. But many components are needed in this traditional solution and it's hard to decrease the cost.